This invention relates to the remote sensing of a gas in the atmosphere employing a radar transmitter for exciting the gas with microwave energy and a radar receiver for detecting microwave energy emitted by the excited gas. More particularly, gas seeps from subsurface formations are identified as to size and concentration as an indication of the possible presence of subsurface hydrocarbon-bearing formations.
Various sources emit different species of gases. Aircraft, automobiles, and other type motor vehicles emit gasoline and fuel oil vapors into the atmosphere. Commercial and industrial users of fuel emit gaseous pollutants or contaminants into the atmosphere. Storage facilities and pipelines to and from such facilities may be leaking gases into the atmosphere. Subsurface geological structures containing hydrocarbon deposits may be fractured or faulted, thereby permitting hydrocarbon gas to seep into the atmosphere.
Several different methods and systems have been developed for locating various species of gases in the atmosphere. One such method and system employs a remote sensing technique wherein a radar transmitter emits a beam of electromagnetic energy at a microwave frequency. The molecules of certain species of gases that might be found in the atmosphere are irradiated by the beam of microwave electromagnetic energy. This irradiation excites the gas molecules to new molecular rotation states from which they emit electromagnetic energy at characteristic molecular resonance frequencies. This electromagnetic energy emitted by the gases is detected by the radar receiver and correlated with known molecular resonance frequencies for certain species of gases to identify the particular species of gas that has been irradiated by the electromagnetic energy from the radar transmitter.
Molecular configurations of certain species of gases are inherently associated with specific molecular resonance frequencies in the microwave energy spectrum. At temperatures above absolute zero, the molecules rotate about some axis defined within the molecule. The molecules are not free to rotate at any arbitrary rate but take on rotations at only certain unique and particular rates. Rotational energy of molecules therefore depends upon the particular rotational-energy state of the molecules. At thermal equilibrium, molecules of a particular species of gas will not all exist at the same rotational-energy state. If the configuration of a molecule is such that its average electrical charge is asymmetrically distributed, it is said to possess an electric dipole moment. Only those molecules having such a dipole moment can change their rotational-energy states through interaction with an incident electromagnetic field. Certain species of gases exhibit such molecular configuration and can therefore interact and exchange rotational energy with an incident electromagnetic field. However, each of such species has its own unique set of possible interaction rotational rates. By knowing such rates for a particular species, electromagnetic energy can be radiated toward and absorbed by the gas in the form of increased rotational energy. A particular species of gas can therefore be detected in a remote sensing operation by employing a radar transmitter at microwave frequencies to excite the gas molecules to new rotational-energy states and a radar receiver to detect the resulting electromagnetic radiation emitted by the excited gas molecules at their characteristic molecular resonance frequencies in the microwave energy spectrum.